The present invention relates to functionally graded materials. Functionally graded material refers to a class of materials that have graded compositions within their microstructure. The graded compositions results in graded mechanical and physical properties and functionality, which may be desirable for commercial applications.
Cemented tungsten carbide is a composite material of tungsten carbide embedded in a cobalt matrix. (Such cemented tungsten carbide materials are often abbreviated as “WC—Co” or “WC—Co materials.”) Typical compositions of cobalt metal range from 3 to 30 percent by weight. Unless otherwise specified, the concentrations expressed herein are weight percent amounts. Cemented tungsten carbide materials have unique properties compared to metal alloys or ceramic materials. For example, WC—Co has much higher hardness, wear resistance, and strength than steel alloys, but much lower fracture toughness than steel alloys. When compared to ceramic materials, WC—Co materials have much higher fracture toughness at equivalent or better hardness and wear resistance levels. Because of their unique mechanical properties, cemented tungsten carbide materials are used in a wide range of industrial applications including metal cutting, mining, oil and gas exploration, and many applications requiring extreme wear resistance.
The applications of cemented tungsten carbide are limited, however, by its relatively low fracture toughness. Chipping and fracturing are the leading causes for degradation or premature failures of cemented tungsten carbide tools. In real life engineering applications, one is forced to trade-off between the wear resistance and fracture toughness. In other words, the fracture toughness is improved at the expense of hardness and wear resistance, and vice versa.
It is therefore highly desirable to improve the fracture toughness of cemented tungsten carbide materials while maintaining their superior wear resistance. The approach of “functionally graded materials” is a viable approach for achieving this goal. In a functionally graded cemented tungsten carbide, the cobalt content of the composite is graded from one surface to another surface or from one reference position to another reference position within a part. Because the wear resistance and toughness of WC—Co materials depend on their cobalt content, the cobalt gradient produces graded properties. For example, a component made of WC—Co material may have 6 percent of cobalt on its surface, but the cobalt content increases gradually as a function of the depth from the surface to the interior of the component until it reaches 16% and then it levels off so that the bulk of the component has 16% Co.
A property gradient of WC—Co material may also be achieved by varying tungsten carbide (WC) grain sizes. However, it is very difficult, if not impossible, to vary grain sizes continuously. Therefore, property gradation achieved by varying grain sizes is almost always non-continuous.
Although it is widely recognized that a graded structure as described above is desired, there is to date no satisfactory manufacturing method that produces such materials with continuous gradation.
Cemented tungsten carbide is usually manufactured by liquid phase sintering (“LPS”) techniques. Typical sintering temperatures range from 1320° C. to 1460° C. At the sintering temperature, cobalt phase becomes liquid. The formation of liquid is necessary to obtain porosity free materials. The Co liquid phase has a limited solubility for the elements W and C according to the WC—Co ternary phase diagram.
The liquid phase sintering process cannot be used directly for making the WC—Co with graded cobalt compositions because the liquid phase cobalt homogenizes during sintering. Any initial gradient of cobalt content prior to sintering, which can be built-in through various powder compaction and shaping techniques, is eliminated during sintering. The final material is not graded.
A logical approach that has often been proposed is to sinter the material at solid state. But solid state sintering does not fully densify WC—Co material. There is usually >1% by volume of porosity remaining after solid state sintering. Such porosity levels significantly degrade desired mechanical properties, rendering the material unacceptable. It is often suggested to eliminate the remaining porosity by using high pressure consolidation processes such as hot isostatic pressing (HIP) or rapid omnidirectional compaction (ROC). Although it is plausible that these high pressure processes fully densify the materials, they add to manufacturing costs considerably (>40%). In addition, the mechanical properties of materials made by high pressure consolidation processes at solid state are not comparable to those of WC—Co materials made by liquid phase sintering.
In short, neither conventional LPS nor solid state sintering processes satisfactorily produce functionally graded WC—Co with continuous gradation. A new method is required.
Two known patents disclose methods for creating graded compositions in cemented tungsten carbide, namely U.S. Pat. No. 5,541,006 (the “'006 patent) and U.S. Pat. No. 6,896,460 (the “'460 patent”). Both the '006 patent and the '460 patent are expressly incorporated herein by reference. However, the methods disclose in these two patents have significant limitations.
For example, the '006 patent teaches a method that creates a graded structure by using two layers that have different magnetic saturation numbers. Measuring magnetic saturation is a known technique in the industry as a non-destructive means to get a relative indicator of carbon level of the material. However, the carbon content variations, as measurable by magnetic saturation numbers, are rather small. Accordingly, the cobalt content gradient created by the method of '006 patent is also rather small—i.e., often in the 1-2% range. In turn, because the cobalt gradient is small, the gradient of mechanical properties in the resulting material will also be small. The fact that only a small gradient of desired mechanical properties is possible under the '006 patent means that this method for preparing functionally graded tungsten carbide materials is inflexible and will likely have few commercial applications.
Likewise, the '460 patent teaches a completely different processing method whereby a graded structure is created through a carburizing treatment-i.e., through a post-sintering heat treatment in a carbon rich atmosphere. However, this extra heat treatment step is expensive and very inefficient. Moreover, this method of production has severe limitations with respect to the depth of the graded zone in a component and the range of graded compositions. In fact, the '460 patent specifies the depth of graded layer to be less than 500 microns, which may not be acceptable for many commercial applications.
Accordingly, while the '006 patent and the '460 patent provide some methods for producing functionally graded materials, these methods are severely limited. A new type of method is needed that is cost effective, more flexible, and will create a wider range of graded microstructures and/or a wider range of properties. Such a new method is disclosed herein.